There presently exists fiber optic sensors which utilize optic fibers and proximity probes which use light transmitted from a light source by way of a bundle of these light conducting optical fibers to a target. The light is reflected off the target and returned along other optical fibers in the bundle back to a light intensity sensor. The intensity of the returning light is a function of the distance between the proximity probe tip and the target, so the output of the light sensor can be a very precise measure of that distance. Typical examples of such sensors can be found in U.S. Pat. No. 3,327,584 issued June 27, 1967; No. 3,940,608 issued Feb. 24, 1976; and 4,247,764 issued Jan. 27, 1981, all hereby incorporated by reference.
Initially, the gap measuring instrument is normalized such that the optic peak of the characteristic curve for that probe corresponds to a 100% or full scale output calibration.
Because of variations in certan operating parameters, particularly reflectivity of the target, transmission efficiency of the fiber optic bundle, and other factors which influence the intensity of the light transmitted and reflected, it is necessary to readjust the instrument, by adjusting light source intensity if any one of these factors are changed during operation. Changing the relative position between the fiber optic bundle tip and the target is necessary to determine if reflectivity variations have affected the instrument, but the position change itself will not compensate for the reflectivity change. Although provisions for this readjustment may be included in the device, they usually are expensive, limited in range, or require special probes. This of course is aside from the situation in certain applications where the need to readjust the instrument for each use is more than merely cumbersome, but impossible, thereby limiting the application of such devices. This is particularly evident in situations where the instrument is in an inaccessible location or where its readings must be available continuously or at unpredictable intervals. Other situations require the permanent mounting of the instrument probe relative to a target.
There exists a need to have a system not requiring readjustment heretofore necessary whenever the operating parameters, particularly the reflectivity of the target changes.